1. Field of the Invention
The present invention relates to an optical scanning apparatus, and an image forming apparatus such as a copying machine, a facsimile, a printer or a multifunctional machine having functions thereof, which are equipped with the optical scanning apparatus.
2. Description of the Related Art
In an image forming apparatus including a copying machine, a facsimile, a laser printer, or a multifunctional machine, there is an increasing tendency in the number of scanning beams used for a scanning optical system to meet demands for high speed, high resolution, and wide writing width of the apparatus. A color image forming apparatus including a plurality of optical scanning apparatuses using multiple beams has also been commonly used.
As a light source for multiple beams installed in the optical scanning apparatus, there are known a semiconductor laser array (hereinafter, referred to as an “LD array”) in which light emitting points are arrayed one-dimensionally at predetermined intervals, a vertical cavity surface emitting laser array (hereinafter, referred to as a “VCSEL”) in which light emitting points are arrayed two-dimensionally at predetermined intervals, etc. Among them, for the former laser array, also use is made of a system in which light fluxes emitted from a plurality of LD arrays are combined using a beam combining prism or the like to increase the number of beams, and this system may be considered to use a light source in which light emitting points are arrayed two-dimensionally as with the latter VCSEL.
On the other hand, in general, as the number of beams of the light source is increased, an allowable level of a scanning line interval error or scanning line interval deviation between scanning positions of scanning lines formed on the scanned surface becomes severer. If the error or deviation increases, then periodic streak and/or contrasting density unevenness or irregularity (referred to as banding) is easily perceptible to the eye in a periodic image such as a horizontal line image or a halftone image, thereby leading to the cause of deterioration in image quality. In addition, in the color image forming apparatus, if so, it may lead to the cause of color deviation (registration shift) of the colors, so that image quality deteriorates.
To this end, for instance, Japanese Patent Application Laid-open No. H4-101112, Japanese Patent Application Laid-open No. H8-015625, and Japanese Patent Application Laid-open No. 2008-076712, there have been proposed adjusting means capable of maintaining good beam pitch and beam diameter that are formed on a photosensitive material serving as a scanned surface, through the movement in an optical axis direction or the rotation around the optical axis of a cylinder lens, so as to make a scanning line interval (beam pitch) formed on the optical scanned surface to be a target interval.
Generally, in a multibeam optical system, since an interval of the beams situated on an image plane can be narrowed by decreasing the arrangement interval of the light sources, the multibeam optical system is suitable for high-density writing. However, in reality, as the intervals of the light sources are decreased, the optical system's lifetime is more shortened due to heat generation, and the optical system is more vulnerable to thermal and electrical crosstalk. For this reason, there is a limitation in decreasing the intervals of the light sources.
With respect to adjusting means for maintaining an appropriate beam interval and an appropriate beam diameter, Japanese Patent Application Laid-open No. H4-101112, Japanese Patent Application Laid-open No. H8-015625, and Japanese Patent Application Laid-open No. 2008-076712 do not have any disclosure of adjustment to the influence of change in temperature.